An attempt will be made to investigate the initial rate kinetics of brain hexokinase at its intracellular concentration. What is known about the kinetics of the enzyme come from studies of hexokinase at 1 nM concentrations. This is far below the level of enzyme found in brain tissue, and the possibility exists that in the physiological concentration range, the properties of hexokinase may change drastically. These experiments will be coupled with studies involving the mode of regulation, i.e., effects of nucleotides, glucose-6-P, and inorganic phosphate, of hexokinase.